1. Field of Invention
The present invention relates to an extreme ultraviolet (EUV) mask and a lithographic process using the EUV mask. More particularly, the present invention relates to an EUV mask and an EUV lithographic process using the EUV mask at a grazing incident angle of an EUV source.
2. Description of Related Art
In an EUV lithographic process of a prior art, a multilayer reflection mask is used for developing the patterns to the wafers. FIG. 1 illustrates a structure of a multilayer reflection mask 110 of the prior art. The multilayer reflection mask 110 in FIG. 1 includes a silicon or glass substrate 102 and a reflective multilayer film 104 is formed on the substrate 102. The reflective multilayer film 104 includes forty pairs of molybdenum and silicon, wherein the molybdenum layers are each about 3 nm thick and the silicon layers are each about 4 nm thick. A capping layer 106 including an amorphous silicon layer having a thickness of about 7 nm is formed on the reflective multilayer film 104 to protect the reflective multilayer film 104 from oxidation. Finally an absorbing layer 108 including silicon oxide or silicon nitride is formed on the capping layer 106.
FIG. 2 illustrates the process of EUV lithography of the prior art. In FIG. 2 an incident EUV 204 emitted from an EUV source 202 irradiates on the surface of the multilayer reflection mask 110 at an incident angle 206. The incident angle 206 denotes the angle between an incident direction of the incident EUV 204 and the surface paralleled to the capping layer 106 or the absorbing layer 108. A portion of the incident EUV 204 irradiating on the capping layer 106 is reflected with high reflectivity in a range of about 70% to about 80%, and the other portion of the incident EUV 204 irradiating on the absorbing layer 108 is absorbed with high absorption of about 90% or more. Therefore, about 70% to about 80% of the incident EUV 204 is transferred into a highly-reflected EUV 208 and about 10% or less of the incident EUV 204 is transferred into a lowly-reflected EUV 210. The reflected EUV 212 including the highly-reflected EUV 208 and the lowly-reflected EUV 210 is transmitted onto a photo resist layer 214 on the surface of the wafer 216. Therefore the patterns on the surface of the multilayer reflection mask 110 are developed to the surface of the wafer 216 by the highly-reflected EUV 208.
In order to achieve high reflectivity on the reflective multilayer film 104, the incident angle 206 is set in a range of about 80 degree to about 89 degree, that means the incident of the EUV is nearly normal. The individual thin film thickness of the reflective multilayer film 104 are preferably controlled tightly to be within their target thickness ±0.1 nm, in order to ensure constructive interference between the layers to achieve a high reflectivity of about 70% or more. The surface roughness of the substrate 102 is also controlled tightly to be within its target thickness ±0.1 nm. And if defects 112 are formed in the process of forming the reflective multilayer film 104, a lot of the defects 112 are not repairable. When all the limitations are tightly controlled, the final reflectivity of the surface of the mask is in a range of about 70% to about 80%. Consequently, in the prior art, it is hard to fabricate a smooth, defect-free and highly reflective EUV multilayer reflection mask.